It has to be. Anything that aids teacher development is really important - and it could be something as basic as improving boardwork to workshopping cool ways to help students better record phrasal verbs. At the English Studio in London, we try to have around three seminars or workshops on teachers' day at the end of each term.
Despite the improvements in teaching they may yield, it's also important to show your teachers that you care about their professional development.

Our schools also encouraged us to go to conferences and paid our fees and travel. We were also encouraged to present at conferences and brought home tons of free material and lots of ideas to share with the other teachers.

We also had monthly meetings and at those, in addition to business, we were asked to share an idea we had used in the previous month and to brainstorm about problems we were encountering with students, schedules, materials and so on.

I think it depends on how much time you have on your schedule. During normal or quiet periods we have a teachers' meeting every week (30 min) to specifically discuss the students in our classes. We also try to hold a one-hour workshop every week. The ws is given in rotation by the teachers themselves; The teachers get experience in presenting to their peers which is 'development' in its own sense. It can be about anything the teacher knows well or has more experience than others. its quite informal and doesn't put the onus of developing the teachers on the DoS alone.

I try to have a weekly teacher's meeting of the Foreign Teachers and we take turns proposing topics to discuss after the meeting. So we'll have a general meeting to cover the various odds and ends that come up and then after wards we'll take a quick break before having a focused discussion on a specific topic. The focused discussion doesn't have a set time limit on it. It can take as little as 15 minutes to a full hour depending upon how much the teachers speak on the subject.

I find this a happy compromise of respecting my teacher's contract time and getting them involved in their own development. Of course most teachers gripe no matter how hard you try to keep their non-teaching hours down, but this way they at least have the room to discuss topics that are pertinent to their classes.

I think you need to consider the level of formality of this type of training carefully. If you have a small and friendly team, formalising such meetings could damage the positive dynamic of the group, in particular between you and the teachers.

Perhaps you could try a more informal, even ad hoc approach. Peer training is also a useful option.

I also think it is important in a small team to consider the individual motivation of the teachers. Some people are really keen to develop professionally and increase their range of skills. Others may be good at what they do but simply wish to maintain their current set of skills.

All in all there's a lot to consider but I think teachers need to feel as if they have the opportunity to develop if they want to.